<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .box {
            /* width: 300px;
            height: 300px;
            background-color: red;
            clip-path: polygon(50% 0%, 0 100%, 100% 100%); */

            width: 0px;
            height: 0px;
            border: 100px solid red;
            border-bottom-color: transparent;
            border-right-color: transparent;
            border-left-color: transparent;
        }
        /* .box {
            width: 0;
            height: 0;
            border: 100px solid transparent;
            border-bottom: 150px solid red;
            position: relative;
        }
        .box::after {
            content: '';
            width: 0;
            height: 0;
            border: 80px solid transparent;
            border-bottom: 120px solid white;
            position: absolute;
            top: 10px;
            left: -80px;
        } */
    </style>
</head>
<body>
    <div class="box"></div>
</body>
</html>